1. Field of the Invention
This invention relates to an improvement of an underwater truss structure. The underwater truss structure comprises a plurality of shafts jointed by a plurality of ball members. More particularly, the underwater truss structure has a plurality of unit structures which are linked together and each of which comprises a plurality of shafts and ball members assembled so as to form a triangle with the ball members respectively positioned at the corners of the triangle.
2. Description of the Prior Art
The underwater truss structure is a light-weight offshore structure which has been invented by this applicant and can substitute for known offshore structures such as a caisson breakwater, tetrapods and the like which owe their wave damping effect to their weight and are inherently heavy. The underwater truss structure can be installed on sandy or weak ground at low cost and can be used also as a man-made gathering-place for fish. The basic structure of the underwater truss structure is disclosed in Japanese Patent Publication No. 52(1977)-3487 (entitled `Underwater Truss Structure`). Further as disclosed in Japanese Patent Publication No. 58(1983)-26443 (entitled `Wave Damping Structure Employing Underwater Truss Structure`), the wave damping effect of the structure can be enhanced by providing brims (disks) on the shafts. Though the wave damping structure disclosed in the patent publication is used as a man-made gathering-place for fish or a substitution for tetrapods, the underwater truss structure can be used for forming various floating structures. An example of such floating structures is disclosed in Japanese Unexamined Patent Publication No. 63(1988)-269799 (entitled `wave damping stable floating structure` and filed by this applicant).
The underwater truss structure is formed by continuously linking a plurality of unit structures each of which comprising a plurality of shafts and ball members assembled so as to form a triangle, e.g., an equilateral triangle or an isoceles triangle, with the ball members respectively positioned at the corners of the triangle. The shaft comprises a compression-resistant tubular member and a tension-resistant tension rod extending inside the tubular member in the axial direction thereof. The ball member connects the shafts at their ends. When equilateral unit structures are three-dimensionally assembled into an underwater truss structure, twelve shafts are fitted in each of the ball members positioned in the interior of the structure. That is, six shafts are radially connected to the ball member at intervals of 60.degree. in a plane passing through the center of the ball member, and three shafts are connected to the ball member at regular intervals to radially extend upward at 60.degree. to the plane and another three shafts are connected to the ball member at regular intervals to radially extend downward at 60.degree. to the plane.
The wave damping effect of the underwater truss structure used as a wave damping structure or a wave trap can be explained as follows. That is, when waves pass through the underwater truss structure, motion of the waves is disturbed by the resistance due to the shape of the structure and is led to a turbulent state depending on the volume of the underwater truss structure. Turbulence is a flow in which swirls of various sizes are mixed in confusion, and each of large swirls is divided into smaller swirls and each of the smaller swirls is divided into further smaller swirls due to the shape of the structure. The kinetic energy of the swirls which is transmitted in the course of such stepwise divisions from a large swirl to smaller swirls (in the course of cascade type divisions) is finally converted into other energies such as heat in the region where the viscous friction of water prevails, and vanishes from the fluid system.
That is, the underwater truss structure exerts wave damping effect by contacting wave fluid and disturbing it to lead it into a turbulent state, thereby promoting cascade type divisions of swirls and converting the kinetic energy of waves into other energies such as heat.
Accordingly if the cascade type divisions can be promoted more effectively, more kinetic energy is consumed due to the internal resistance of the fluid itself and the resistance to which the structure is subjected is reduced, which is advantageous in view of the strength of the structure. Further the wave damping capacity is increased and accordingly the structure can be smaller in size, whereby cost of the structure can be greatly lowered.
The conventional underwater truss structure which is formed by combination of regular triangular pyramid or regular square pyramid three-dimensional unit structures solely consisting of shafts and ball members (will be referred to as "the normal three-dimensional unit structure", hereinbelow) has a contact area per unit volume with the flow of water which is insufficient to effectively promote the cascade type division and accordingly must be large in scale in order to obtain a desired wave damping effect. Previously, this applicant has proposed to provide a brim on the shaft in order to increase the contact area with the flow of water of the structure (Japanese Patent Publication No. 58(1983)-26443). That is, the underwater truss structure disclosed in the patent publication has three-dimensional unit structures each comprising a plurality of brimmed shafts and a plurality of ball members (will be referred to as "the brimmed three-dimensional unit structure", hereinbelow). The brims on the shafts contribute to large increase in the contact area with fluid per unit volume and to large increase in the wave damping capacity, whereby the underwater truss structure can be small in scale.
In order to improve efficiency in assembly of the underwater truss structure, the structure must be able to be easily assembled.
Further since the underwater truss structure is generally used offshore, parts of the structure are subjected to very large compression and/or tension, and accordingly the structure must be as great as possible in strength.
As described above, the shafts are connected by way of the ball member, the shafts and the ball member must be connected so that both the compression force imparted to the tubular member of each shaft and the tensile force acting on the tension rod of each shaft are transmitted to other shafts. For this purpose, the tension rod of the shaft which resists tensile force must be firmly fixed to the part of the ball member which resists tensile force so that they are not moved away from each other, and at the same time the end of the tubular member of the shaft which resists compression force and the part of the ball member which resists compression force must be kept in a state where they are in abutment against each other not to adversely affect the connection between the tension rod and the part of the ball member which resists tensile force when a compression force is applied thereto. In this specification, the expression "the end of the tubular member and the part of the ball member which resists compression force are in abutment against each other" should be broadly interpreted to include states where they are directly abut against each other and where they are abut against each other with another member sandwiched therebetween, and such a state of connection will be referred to as "the state of proper connection", hereinbelow.
However since the shaft and the ball member are substantially heavy in weight and at the same time, the distance between the ball members comes to be fixed as the assembly progresses, great difficulties are encountered in three-dimensionally assembling the shafts and the ball members into the underwater truss structure while keeping the state of proper connection between the shafts and the ball members. Thus there is a demand for an underwater truss structure which can be assembled easily and efficiently.
Further there are some problems in the brimmed three-dimensional structure. That is, since the brim is provided on the shaft to extend in the direction substantially perpendicular to the axis of the shaft, bending moment as well as an axial force acts on the shaft due to the pressure of water flow acting on the face of the brim if the brim is fixedly connected to the shaft. Accordingly, to widen the area of each brim and/or to increase the number of brims to be provided on each shaft in order to increase contact area with the flow of water can result in excessively large bending moment and axial force acting on the shaft as well as increase in cost and has been difficult to put into practice.
In order to overcome such problems, this applicant has proposed to resiliently connect the brim to the shaft so that the pressure impact imparted to the brim from the flow of water is damped before transmitted to the shaft. See, for instance, Japanese Unexamined Patent Publication No. 1(1989)-180530. However with this arrangement, there still remain problems of enhancement of the brim against the pressure of the flow of water and prevention of axial displacement of the brim relative to the shaft.
Further, in addition to these problems, since the number of brims which can be provided on one shaft is limited, the contact area per unit volume with water can be increased to only a limited extent solely by the brimmed structure.
Thus there has been a demand for a new type three-dimensional unit structure in which the contact area per unit volume can be larger than in the normal three-dimensional unit structure and at the same time which is in a form different from the brimmed three-dimensional unit structure and can be combined with the brimmed three-dimensional unit structure to further increase the contact area per unit volume, thereby more effectively promoting cascade type divisions of swirls.